1. Field of the Invention
The present invention relates to a brake system for a motorcycle. It particularly relates to a brake system for a motorcycle which causes hydraulic pressure generated by an electrically-operated actuator to function on wheel braking mechanism depending on driving conditions and a braking operation of the motorcycle. The brake system can simultaneously cause a pseudo-reaction force to function on a brake operation apparatus.
2. Description of the Background Art
A brake system employing what is called a by-wire method (hereinafter referred to as a “by-wire method”) is known in the art. According to this type of brake system, an amount of operation of a brake operation lever, such as a hand lever or foot pedal, is detected electrically. Based on the detected value, the wheel braking mechanism, such as a wheel caliper, is operated by hydraulic pressure which has been generated by a hydraulic pressure modulator. An example of this type of brake system is described in Japanese Patent Laid-open Official Gazette No. Hei. 5-39008. The brake system disclosed in Japanese Patent Laid-open Official Gazette No. Hei. 5-39008 has the following schematic configuration.
Specifically, with regard to this type of brake system, a hydraulic pressure modulator which generates hydraulic pressure by use of an electric pump (electrically-operated actuator), a reservoir, a control valve and the like are merged and connected with a main brake line connecting wheel cylinders (wheel braking mechanism) of the respective wheels and a master cylinder which moves in response to a brake pedal (brake operation lever). In addition, a normally-open-type electromagnetic on-off valve is provided in a position in the main brake line. The position of the electromagnetic on-off valve is located toward the master cylinder from a portion in which the main brake line and the hydraulic pressure modulator are connected with each other. The electromagnetic on-off valve is a fail safe device. The electromagnetic on-off valve is energized, and blocks the path between the electromagnetic valve and the master cylinder, while a regular braking operation is performed. In addition, hydraulic pressure is generated by the hydraulic pressure modulator. The hydraulic pressure is generated corresponding to a detection value which has been found by electrically detecting an amount of operation of the brake pedal, and to other driving conditions of the vehicle.
Furthermore, this type of brake system is provided with a reaction force modulator. The reaction force modulator causes a pseudo reaction force against the hydraulic pressure to function on the master cylinder corresponding to an amount of operation of the brake pedal, while the electromagnetic on-off valve shuts off the main brake line. Thereby, this type of brake system is designed provide a braking operation which transmits a smooth feel to the toes of a driver in accompaniment to operation of the brake pedal. The reaction force modulator includes a fluid chamber which receives hydraulic fluid which flows into from the master cylinder. The fluid chamber is separated and formed by a piston and a cylinder. The piston is energized by a single coil spring in a direction which causes the piston to reduce the volume of the fluid chamber. In the reaction force modulator, reaction force against hydraulic pressure corresponding to displacement of the coil spring always functions on the master cylinder.
With regard to this conventional type of brake system, however, the reaction force against hydraulic pressure to be generated by the reaction force modulator is provided by the single coil spring. For this reason, it is difficult for this type of brake system to give the driver a braking operation in which the feel is not ragged, and to give the driver a braking operation in which the feel is similar to that generated by a brake system which directly operates a wheel braking unit by use of pressure generated by a master cylinder (hereinafter referred to as a “direct-operation-type brake system”). In other words, with regard to the direct-operation-type brake system, a moderate rise of reaction force continues during a predetermined length of stroke in an initial phase of braking operation, and a relatively steep rise of reaction force is caused along with some degree of damping resistance after the stoke exceeds the predetermined length. However, it is virtually impossible for characteristics of this kind to be reflected on a braking operation feel by reaction force caused by a reaction force modulator having only a single coil spring.
In addition, it is conceivable that use of an electric servo mechanism or the like in the reaction force modulator would reproduce operational reaction force which is similar to that generated by the direct-operation-type brake system. However, forming a reaction force modulator which includes such devices leads to a larger size and heavier weight of the brake system. This is not suitable especially for a motorcycle which is required to be smaller in size and light in weight.
Furthermore, in the aforementioned conventional brake system, an electromagnetic on-off valve (channel switching valve) is installed in the fluid path which connects the master cylinder and the reaction force modulator so that the electromagnetic on-off valve is interposed between the master cylinder and the reaction force modulator. The conventional brake system is designed to cause the electromagnetic on-off valve to control flow of hydraulic fluid into the reaction force modulator. In this system, however, the flow of the hydraulic fluid from the master cylinder into the reaction force modulator, and return of the hydraulic fluid from the reaction force modulator into the master cylinder, are always carried out through the same single path. For this reason, the electromagnetic on-off valve sometimes closes the path before the hydraulic fluid has been completely returned from the reaction force modulator into the master cylinder, depending on operating conditions of the electromagnetic on-off valve. In this case, a feeling of changed braking operation is apprehended the next time the brake system is operated.
With this taken into consideration, it is an object of the present invention to provide a brake system for a motorcycle which provides a feeling of braking operation which is similar to that given by the direct-operation-type brake system, to be obtained even though the brake system is designed to operate the wheel braking mechanism by use of the by-wire method.